The present invention relates generally to an echo canceller, and more specifically to an efficient adaptive digital echo canceller for eliminating the echo interference to a modem operating in a full-duplex environment.
In recent years, multi-tone modulation has become very popular for reliable data communication with the highest possible data rate, and has been proven quite successfully. The asymmetric digital subscriber line (ASDL) standard adopts the discrete multi-tone modulation (DMT) technology to provide high speed data transmission on a band-limited channel.
In general, an ADSL system operates over a twisted-pair telephone loop. Because of imperfect impedance matching of a twisted-pair loop over a wide frequency band, an echo may be created when a transmitted signal is passing through and partially reflected back from a hybrid circuit in the ADSL system. The echo is undesired leakage of transmitted signals from a transmitter and feedback into a near-end co-located receiver. It causes great interference to the intended signal reception.
It is possible to separate the bandwidth in the two directions of a full-duplex transmission system over a twisted-pair loop and thus reduce the echo by means of frequency-division multiplexed (FDM) technique. However, the data rate is decreased due to the limited bandwidth. In many cases, it is desirable that an ADSL modem have the option of using overlapped bandwidth in the two directions of transmission to increase the aggregate data rate and improve the transmission throughput. Therefore, an efficient echo canceller is necessary to eliminate the echo interference in a full-duplex modem that has frequency-overlapped option for data transmission.
An echo canceller can be accomplished using either an analog circuit or a digital circuit. FIG. 1 illustrates the application of a traditional digital adaptive echo canceller in a data transmission system that includes a transmit path 101, a transmit filter 102, a digital-to-analog converter (DAC) 103, a hybrid circuit 104, an analog-to-digital converter (ADC) 105, a receiver filter 106 and a receiver path 107. In most cases ,an echo canceller 108 is installed in parallel with the echo path that includes the transmit filter 102, the digital-to-analog converter (DAC) 103, the hybrid circuit 104, the analog-to-digital converter (ADC) 105, and the receiver filter 106. The echo canceller 108 estimates the echo channel and produces an echo replica that can be subtracted from the received signal before it enters the receiver path 107.
Several echo cancellation techniques have been presented in the prior arts. An example of conventional echo cancellers uses a finite response filter (FIR) with long taps to model the echo channel and cancel the echo in time domain. To efficiently eliminate the echo, the conventional echo canceller has to update its FIR tap coefficients very frequently in time domain and the computational complexity is very high. It does not present an effective solution to the ADSL applications. Another example of conventional echo cancellers is implemented purely in frequency domain based on the dual relationship between frequency domain and time domain. The implementation of this type of echo cancellers requires a high-order fast Fourier transform (FFT) or inverse fast Fourier transform (IFFT) that takes extra effort and cost.
J. M. Cioffi and J. Bingham published a paper titled xe2x80x9cA Data-Driven Multitone Echo Cancellerxe2x80x9d in Vol. 42, No. 10, IEEE Transactions on Communication, October 1994. An echo canceller design containing a frequency-domain canceller for coefficient setting and updating and a time-domain canceller for echo synthesis is disclosed in the paper. The design requires very frequent FFT/IFFT operations between frequency and time domains. In addition, the assumptions in achieving their design can not be satisfied in long echo channels.
Minnie Ho, J. Cioffi and J. Bingham further presented another echo canceller in a paper titled xe2x80x9cDiscrete Multitone Echo Cancellationxe2x80x9d in Vol. 44, No. 7, IEEE Transactions on Communication, July 1996. The echo canceller also relies on both time and frequency domain cancellation to cancel echoes. The innovative structure can reduce significantly the computational complexity of an echo canceller for ADSL or similar applications in the numbers of multiplication and addition in some cases. However, it does not really gain much advantages for an ASIC implementation of the architecture from the real time processing point of view when the echo channel is not short enough, is which case the time domain tail emulation in the architecture of ASIC still needs a lot of numerical operation.
The present invention has been made to meet the requirement of echo cancellation in a full-duplex ADSL application. The primary object is to provide an architecture for an efficient adaptive echo canceller. According to the invention, the echo canceller comprises a frequency domain update block, a far-end signal estimation block and a time domain echo cancellation block.
The frequency domain update block generates a frequency domain echo channel estimate of which an inverse fast Fourier transform is computed to form a synthesized time domain echo channel. The far-end signal estimation block generates an estimate of a frequency domain target channel i.e. the combination of the loop channel and the time-domain equalizer (TEQ) for synthesizing a far-end signal which should be subtracted from residual echo estimate at show time to obtain an error term signal. The error term signal is used for updating and returning the frequency domain echo channel estimate in the frequency domain update block at show time. The time domain echo cancellation block synthesizes an echo replica using the estimated time domain echo channel and subtracted the echo replica from a received signal.
Another object of the present invention is to provide a method for training the adaptive echo canceller. Accordingly, the adaptive echo canceller has a training mode to approach the expected system performance. In the training mode, the echo canceller starts with an echo channel training state in which the tap coefficients of the echo channel are trained. In the invention, the system uses periodic frames such as PRU (persudo random upstream codes) for this training. After the echo channel estimate has been trained, the training mode continues with a target channel training state in which the frequency domain target channel estimate is trained.
It is also an object of the present invention to provide a returning method for updating the target channel estimate and echo channel estimate. After the training mode, the echo canceller is switched to an operation mode. The echo canceller continues the time domain echo synthesis and echo cancellation frame by frame. When a synchronization frame is received, both frequency domain echo channel estimate and target channel estimate are returned to improve the system performance.
Other features and advantages of the invention will become better understood from the following description of the invention which refers to the accompanying drawings.